In known wireless communication systems, an equalizer is used in the receiver in order to equalize the received signal prior to further processing in the receiver.
However in a mobile communication system, a mobile may be moving quickly or have a carrier offset, such that the signal received in the receiver has a frequency shift.
This frequency shift needs to be removed in the receiver for the received signal to be properly processed. More particularly, the frequency shift should be removed prior to equalization of the received signal.
Existing wireless communication systems utilize techniques for removing this frequency shift. In one example, a Kalman filter based recursive estimator is used to remove the frequency shift and equalize the received signal.
The problem is to be able to receive a signal from a fast moving mobile or from a mobile where there is a carrier offset. The received signal has a frequency shift, which should be corrected.
The frequency error is usually estimated from the samples of the received samples. This estimate is typically worse when the channel quality is poor and gets better as the channel quality improves. When there is no frequency shift and the channel conditions are near the sensitivity level of a receiver, a frequency correction algorithm should not degrade the performance of the receiver. The problem of frequency error correction has become a potential problem in the case of EDGE wireless systems in particular, and as such improvements are desired.
A Maximum-Likelihood method for frequency estimation is defined in M. Luise and R. Reggiannini, “Carrier Frequency Recovery in All-Digital Modems for Burst-Mode Transmissions, IEEE Trans. On Comm., February/March/April 1995. Also Australian Patent No. AU 664626 describes a system where the Doppler correction is based to change in TOA (Time of arrival).
It is an aim of the present invention to provide an improved technique for estimating and removing the frequency shift in a received signal in a communication system such as a wireless communication system.